linux-kernel - Re: [PATCH v6 01/12] KVM: x86: fix possible infinite loop caused by reexecute

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Message-ID: <5034CA2D.30903@redhat.com>
Date:	Wed, 22 Aug 2012 15:01:49 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v6 01/12] KVM: x86: fix possible infinite loop caused
 by reexecute_instruction

On 08/21/2012 05:57 AM, Xiao Guangrong wrote:
> Currently, we reexecute all unhandleable instructions if they do not
> access on the mmio, however, it can not work if host map the readonly
> memory to guest. If the instruction try to write this kind of memory,
> it will fault again when guest retry it, then we will goto a infinite
> loop: retry instruction -> write #PF -> emulation fail ->
> retry instruction -> ...
> 
> Fix it by retrying the instruction only when it faults on the writable
> memory
> 
> Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
> ---
>  arch/x86/kvm/x86.c |   12 +++++++++++-
>  1 files changed, 11 insertions(+), 1 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index fb0d937..704680d 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4473,6 +4473,7 @@ static int handle_emulation_failure(struct kvm_vcpu *vcpu)
>  static bool reexecute_instruction(struct kvm_vcpu *vcpu, gva_t gva)
>  {
>  	gpa_t gpa;
> +	pfn_t pfn;
> 
>  	if (tdp_enabled)
>  		return false;
> @@ -4490,8 +4491,17 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, gva_t gva)
>  	if (gpa == UNMAPPED_GVA)
>  		return true; /* let cpu generate fault */
> 
> -	if (!kvm_is_error_hva(gfn_to_hva(vcpu->kvm, gpa >> PAGE_SHIFT)))
> +	/*
> +	 * Do not retry the unhandleable instruction if it faults on the
> +	 * readonly host memory, otherwise it will goto a infinite loop:
> +	 * retry instruction -> write #PF -> emulation fail -> retry
> +	 * instruction -> ...
> +	 */
> +	pfn = gfn_to_pfn(vcpu->kvm, gpa_to_gfn(gpa));
> +	if (!is_error_pfn(pfn)) {
> +		kvm_release_pfn_clean(pfn);
>  		return true;
> +	}
> 
>  	return false;
>  }
> 

Good catch.  Did this actually happen or did you find it by code inspection?

-- 
error compiling committee.c: too many arguments to function
--
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